Products driven by batteries have a general disadvantage compared to corded AC-driven products since battery voltage is reduced during a discharge cycle of a battery. In a battery powered vacuum cleaner including an electric motor, a decrease of battery power during the discharge cycle of the battery will result in a decrease of the suction power of the vacuum cleaner. Generally, the maximum inlet power of a fan unit of a vacuum cleaner powered by battery will decrease with 50% or more during the discharge cycle of the battery for a given motor current. Also, ageing of the battery will impact the power output such that the initial and highest power available for an aged battery can be reduced with more than a third as compared to a brand new battery. The electrical power supplied to motor is further dependent on a state of charge of the battery; i.e. at which point in a charge cycle the battery is when the charge cycle is interrupted. A further disadvantage is that the rate at which the voltage drops increases as the charge remaining in the battery decreases.
In order to limit the effect of battery voltage reduction over a discharge cycle in the art, feedback control has been proposed. In EP 2 064 808, this is accomplished by measuring the motor voltage and controlling the motor voltage towards a predetermined constant target value. Thus, instead of having a steady decrease in suction power of the vacuum cleaner during the discharge cycle of the battery, the vacuum cleaner is controlled to operate at a constant suction power which is lower than maximum available suction power at any point during the discharge cycle of the battery, until the battery no longer can deliver a supply voltage for achieving the set constant suction power, wherein the motor is instantly switched off.
From a user perspective, this approach may not be desired since there is a great risk that the predetermined constant value from a practical viewpoint is set too low. In EP 2 064 808, there is a clear trade-off to be made between an extended battery discharge cycle with a maintained constant suction power, and a less extended discharge cycle with higher constant suction power.